1. Field of the Invention
The present invention relates to communications systems, and in particular, to transmissions over imperfect channels.
2. Description of Related Art
When a radio frequency communication signal is transmitted, it usually does not follow the straight line path from the sender to the receiver—unless, of course, the two are physically connected, say by a telephone wire. Instead, the signal typically radiates in all directions from the sender. Thus, while some of the radiated signal actually travels a straight line path to the receiver, other portions of the radiated signal may take a more circuitous route, perhaps being deflected off of some physical body en route to the receiver. Consequently, the different portions of the signal that eventually reach the receiver may not arrive simultaneously. This creates a type of distortion known as multi-path distortion. Multi-path distortion is characterized by the fact that multi-path copies of a signal arrive at a receiver, offset in time from each other.
Moreover, these various versions of the signal, and additional signals from sources other than the sender, may introduce interference that tends to distort the signal. The clarity of the signal may be further compromised if the receiver is moving relative to the sender.
A type of radio frequency communication signal that is often utilized is a spread spectrum signal. Spread spectrum signals differ fundamentally from AM or FM radio signals. On the one hand, AM and FM signals have their signal information encoded in a fixed bandwidth assigned to a radio frequency. On the other hand, spread spectrum signals spread the energy contained in a signal across the entire radio frequency spectrum. In addition, in the spread spectrum context multiple signals may co-exist across the radio frequency spectrum, without interfering with each other.
A spread spectrum signal appears as a slight increase in the noise floor, when received by an uncorrelated receiver such as is used to receive AM and FM signals. The signal is correlated to a prescribed code sequence in order to detect the signal.
In a direct sequence spread spectrum transmitter an information containing carrier is modulated by a repeating code sequence. The speed of the code sequence is the chipping rate—measured in chips per second (cps). At the receiver, information from the signal is recovered by multiplying the transmitted signal by a locally generated replica of the code sequence. The replica of the code sequence is applied to the received signal with a set timing relationship.
The circuitry in the receiver that extracts the desired signal is called a spread spectrum correlator. The correlator may be envisioned as a special type of matched filter. The filter only responds to signals encoded with a code that matches its own code, and the receiver is tuned to another signal by changing codes.
Spread spectrum demodulation has two main steps: removing the spectrum spreading modulation—a process known as dispreading—and demodulating the signal to extract its information content. Despreading is accomplished satisfactorily when timing synchronization of the spreading code between the transmitter and receiver is achieved. Synchronization has two components: the initial acquisition of timing information, and the tracking of the timing. Synchronization, or timing information, is typically transmitted with the information content of the signal, and may be affected by any distortion encountered.
One device that is used to help the receiver to home in on the signal is a training sequence. A training sequence is a predetermined signal that is regularly sent as a sort of reference. Thus, the receiver is set to expect that the training sequence has certain predetermined characteristics. If the receiver receives an uncorrupted training sequence, no adjustment to the receiving unit is necessary. However, if there is some deviation from what is expected in the training sequence, the receiver can be adjusted to correct for the error, thus ensuring that when the actual communication signal is sent, it is received without any problem.
It is desirable to use the characteristics of the transmission channel in providing a system and method of estimating timing information. This is especially desirable in frequency-selective, slowly fading channels.
The problem of synchronization of DSSS communications over frequency-selective, fading channels, has not been completely solved. In frequency-selective, fading channels—unlike AWGN channels—estimates of both the received carrier phases and the amplitudes of the multi-paths must be made, whereas phase and amplitude estimates are not utilized in the simpler AWGN model. Moreover, the mutual interference due to multi-paths further complicates the problem of timing recovery. Therefore, there is a significant need for a method and an apparatus that will solve the problem of synchronization in DSSS communications over frequency-selective, fading channels.
While it is useful to estimate the received time, carrier phases, and amplitudes of the factors listed above, the timing estimate tends to play a dominant role in synchronization. The dominance is because the received time provides important prior information that allows the estimation of the received carrier phases and amplitudes. Therefore, to provide improved synchronization in DSSS communications over frequency-selective, fading channels, there is a real need for a method and an apparatus to estimate the received time.
Thus, the embodiments of the present invention provide a method and an apparatus to estimate the received time, by solving the synchronization problem in DSSS communications over frequency-selective, fading channels.